1. Technical Field
The present invention relates to compositions which are used as polishing slurries in the process of polishing silicon wafers for the semiconductor industry.
2. Background Art
Silicon wafers for the semiconductor industry must possess a high degree of surface perfection before they can be useful in the device fabrication process. These surfaces are universally produced by polishing the wafer with a polishing slurry. Polishing slurries generally consist of a composition which contains a concentration of submicron particles. The part, or substrate, is bathed or rinsed in the slurry in conjunction with an elastomeric pad which is pressed against the substrate and rotated such that the slurry particles are pressed against the substrate under load. The lateral motion of the pad causes the slurry particles to move across the substrate surface, resulting in wear, or volumetric removal of the substrate surface. Ideally, this process results in the selective erosion of projecting surface features so that when the process is completed a perfect plane surface is produced down to the finest level of detail.
The silicon polishing process as practiced in industry consists of two or more steps. In the first, or coarse polish step, gross defects remaining from wafer sawing and shaping operations are removed. The wafer surface appears smooth and specular but still contains numerous minute defects. These defects are removed by subsequent final polish steps which remove little material from the surface but act to polish away the surface defects. which remove little material from the surface but act to polish away the surface defects. The present invention relates to solutions which are particularly useful for this final polish process.
The number and allowable size of surface imperfections remaining after polishing has been continuously decreasing with time for many years. Some of the most critical material specifications are the surface metals content, the front surface microroughness and the total particles per unit area.
The metal ion concentration on a wafer surface may be determined by several conventional means of chemical analysis. The most popular non-destructive technique used in the semiconductor industry is X-Ray Fluorescence (XRF). While there are many possible means of introducing metal ions into the wafer surface, it is generally agreed that materials and solutions which come in contact with the wafer surface should have the lowest possible metal ion contents to prevent possible contamination.
The measurement of surface roughness and surface defect/particle concentrations on polished wafers is generally performed by a scanning light scattering detector. Various models; e.g. Censor ANS100, Tencor 6200 and Estek WIS9000; are widely used in the silicon wafer industry. All detectors utilize the same principle of operation, namely that they measure the degree of non-specular reflected light from the wafer surface. A high intensity laser beam is scanned across the surface of the wafer. Non-specular reflected light is collected in an off-axis detector and the signal intensity of this scattered light is analyzed. Surface roughness results in a generalized light scattering of low intensity, generally termed haze. Particles or other discrete surface imperfections produce a more intense scattering which has a point source origin. The intensity of these point sources of scattering is ranked in comparison to that of latex standard calibration spheres of varying sizes. These point sources are generally referred to as Light Point Defects, or LPDs and their distribution is ranked according to Equivalent Latex Sphere sizes which give equivalent scattering intensity. A good general review of wafer measurement technology and terminology is given by P. O. Hahn et al. in an article entitled "The Si--SiO.sub.2 Interface Roughness: Causes and Effects" in C. R. Helms and B. E. Dead, Eds., The Physics and Chemistry of SiO2 and the Si--SiO2 Interface, pp.401-411, Plenum Press, New York (1988) incorporated by reference herein.
From the above, it is clear that a practically useful final polish process for achieving high surface quality levels must be one which does not introduce surface metals onto the substrate surface and produces a low roughness surface free from particles or other sources of light scattering on the wafer surface. It is generally observed that existing prior art final polishing slurries cannot achieve the desired results; existing slurries are either insufficiently pure or do not readily allow haze or LPD targets to be met.
There are numerous background patents on slurries to be used for polishing silicon wafers. The following are considered relevant to the present invention.
Walsh and Herzog (U.S. Pat. No. 3,170,273) describe polishing slurries for silicon wafers consisting of a silica sol having a size between 5-200 nm in a concentration ranging from 2-50% by weight. No criticality of pH, freedom from metal ion contaminants or other additives was taught.
LaChapelle (U.S. Pat. No. 3,429,080) described a polishing slurry for silicon wafers consisting of a dispersion of abrasive particles of a size below 20 microns and an oxidant, wherein the slurry pH ranged from 4.5 to 14. No criticality of pH, solids concentration, freedom from metal ion contaminants or other additives was taught.
Cromwell (U.S. Pat. No. 3,807,979) described a polishing slurry for silicon wafers comprising a quaternary ammonium silicate and precipitated silica. Nothing was taught as to the criticality of solids content, pH, freedom from metal ion contaminants or other solution additives.
Tredinninck et al (U.S. Pat. No. 3,715,842) described a polishing slurry for silicon wafers comprising an abrasive in a slurry concentration ranging from 5-80% solids and a water soluble cellulose compound. Nothing was taught as to the criticality of solids content within this exceedingly large range, pH, freedom from metal ion contaminants or other solution additives.
Basi et al (U.S. Pat. No. 4,057,939) described a polishing slurry for silicon wafers comprising silica particles, sodium carbonate (as a base), an oxidizing agent, and sodium dichloroisocyanurate. Nothing was taught as to the criticality of solids content, pH, freedom from metal ion contaminants or other solution additives.
Payne1 (U.S. Pat. No. 4,169,337) described a polishing slurry for silicon wafers comprising SiO.sub.2 particles and an amine having a concentration of 0.1-5% based on SiO.sub.2 content. A wide variety of amines were disclosed including aminoethanolamine. No limits on SiO.sub.2 concentration were taught and the amine was incorporated in concentration ranges far in excess of what would be required for pH adjustment. Nothing was taught as to the criticality of solids content, pH, freedom from metal ion contaminants or other solution additives.
Payne2 (U.S. Pat. No. 4,462,188) described a polishing solution for silicon wafers comprising silica particles, a water soluble amine in concentration between 0.5-5% and a quaternary ammonium salt or hydroxide. A variety of amines were disclosed, including aminoethanolamine. The preferred ammonium salt or hydroxide disclosed was tetramethyl ammonium chloride or hydroxide. Nothing was taught as to the criticality of solids content, pH, freedom from metal ion contaminants or other solution additives.
Prigge et al (U.S. Pat. No. 4,468,381) described a polishing slurry for silicon wafers comprising solid particles of a concentration between 1-10%, a buffer compound to control pH, an alcohol (preferably glycerols), a polar organic (preferably glycols) and a surface active substance which could include alkyl phenols, alkyl sulfonates, alkyl succinates or polyacrylates. The pH range of activity was restricted to 3-7. Nothing was taught as to the criticality of solids concentrations beyond the wide range described, the incorporation of other salts or the freedom from metal ion contaminants.
Huff (U.S. Pat. No. 4,892,612) teaches the utility of using a silica sol-amine combination in specific proportions to provide high polishing rates at low as-used solids concentration for stock polishing. A lower effective solids limit for the sol-amine combination was .about.1%; a solids content of 0.5% as-used gave degraded results. Nothing was taught as to the incorporation of other salts or other solution additives. In addition, the materials described were utilized in a stock polish process. Nothing was taught as to their potential utility as a final polish slurry.
Sasaki et al (U.S. Pat. No. 5,226,930) described a polishing slurry comprising a dispersion of SiO.sub.2 particles at a concentration between 0.1-10% by weight and with a solution pH ranging from 8-12 which was adjusted by the incorporation of an amine or ammonium hydroxide.
None of the prior art cited or found teaches the criticality of providing a polishing slurry free from metal ions nor do they teach specific compositions which are suitable for a final polishing process which produces a polished silicon surface free from haze or point scattering defects. Indeed almost all of the prior art is exceedingly vague as to the use to which their slurries described may be put and there results obtained therefrom. Of the art examined, only Prigge et al and Payne2 teach the use of their slurries in a final polishing step. Payne2 does not teach the recognition of any special compositional requirements for a final polish slurry; the same slurry formulation is used in both stock and final steps for all examples disclosed.
From the above, it is clear that it would be highly desirable to provide a final polishing slurry free from metal contaminants which was specifically designed to produce surfaces having desirably low haze and point scattering defect levels.